1. Field of the Invention
The present invention relates to a flow meter, and more specifically to a flow meter pickoff assembly and flow meter pickoff adjustment method for nulling a flow meter zero offset.
2. Statement of the Problem
It is known to use Coriolis mass flow meters to measure mass flow and other information of materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flow meters have one or more flow tubes of different configurations. Each conduit configuration may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial and coupled modes. In a typical Coriolis mass flow measurement application, a conduit configuration is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. The vibrational modes of the material filled systems are defined in part by the combined mass of the flow tubes and the material within the flow tubes.
When there is no material flowing through the flow meter, all points along a flow tube oscillate with an identical phase. As a material begins to flow through the flow tube, Coriolis forces cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Sensors are placed at different points on the flow tube to produce sinusoidal signals representative of the motion of the flow tube at the different points. A phase difference of the signals received from the sensors is calculated in units of time. The phase difference between the sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes.
However, there can be inaccuracy in this phase difference. One source of error can come from imperfections in the flowtube apparatus. Another source of error can come from improperly aligned sensor components. Yet another source of error can come from variability in the meter electronics.
One way of detecting inaccuracy in the flow meter is by vibrating the empty flowtube apparatus and measuring the resulting phase difference. This phase difference in the flowtube apparatus, such as for air, for example, is termed a zero offset. Ideally, the zero offset will be zero for a no flow condition (i.e., for air). However, this is usually not the case. Multiple manufacturing tolerances, material variations, improper sensor alignments, and electronic component tolerances can combine to produce a zero offset ranging away from ideal. In addition, the zero offset can be affected by temperature. Unfortunately, the greater the zero offset, the more the zero offset is likely to be affected by temperature.
Although the FCF is currently compensated for these temperature effects, the zero offset is typically not adjustable in a prior art flow meter and the zero offset cannot be mechanically compensated. In the prior art, the problem is typically approached by the user of the flow meter being required to re-zero the flow meter when the temperature changes by more than 20 degrees Centigrade.
Significant work has been done over the years relating to the reduction and/or elimination of zero offset in Coriolis flow meters. The prior art has taken two approaches. The first prior art approach has been to minimize the zero offset of a flow meter through small tolerances and rigorous manufacturing methods. The second prior art approach has been to address the zero offset problem through advanced signal processing, such as modal filtering, compensating for residual flexibility, etc. However, both prior art approaches are costly, complex, and not satisfactorily accurate and successful.